Normal, healthy cells divide to replace those lost or damaged and then stop dividing. Cells normally divide and increase in number in a process called mitosis.
Cancer is a condition where cells multiply. But this multiplying is continuous and not normal. The cells divide and grow out of control. They may grow into nearby tissue or spread to distant parts of the body. Over time, the mass of cancer cells can get big enough to make lumps (also called masses or tumors) that can be felt or seen. But, not all tumors are cancer. Tumors can be benign or malignant:
Are not cancer
Can usually be removed
Don't come back in most cases
Don't spread to other parts of the body, and the cells don't invade other tissues
Are rarely a threat to life
Are cancer and may be a threat to life
Can invade and damage nearby tissues and organs
Can spread or metastasize. Cancer cells break away from a malignant tumor and move through the bloodstream or lymphatic system to form tumors in other parts of the body.
There are general categories of cancer based on the type of cell the cancer starts in. Carcinomas and, more specifically, adenocarcinomas are the most common type:
Carcinomas. Cancers that start in epithelial surfaces. These surfaces are the cells that form the outer surface in the body to line or cover the body's organs, cavities, tubes, and passageways. These cancers form solid tumors.
Adenocarcinomas. Cancers that start in glandular tissues that make mucus or fluid, such as the lung, breast, prostate, or colon. Adenocarcinomas are considered a specific type (subtype) of carcinomas.
Sarcomas. Cancers that start in cells that make up supporting structures, such as bone, muscle, cartilage, fat, or fibrous tissue. These cancers form solid tumors.
Leukemias. Cancers that start in blood cell, most often white blood cells. Because these cancers are in the blood cells, they don't cause tumors.
Lymphomas. Cancers that start in cells of the lymph nodes and lymphatic system. They may cause tumors in lymph tissue.
Brain cancers, nerve cancers, melanomas, and certain testicular and ovarian cancers are other types of cancer categories. They start in different kinds of cells in the body.
Cancers start in a single cell that has been damaged (mutated). That cell is the source of the primary cancer. The cancer is always named for the primary site where the original or first tumor started, such as skin, colon, or breast.
Cancer can spread from where it started (the primary site) to other parts of the body.
The cancer may spread by directly entering nearby tissues.
The cancer may spread throughout the body. This is called systemic spread. The cancer cell may get into and travel through the:
Blood system. Arteries and veins take blood to and from all areas of the body.
Lymphatic system. A network of lymphatic vessels in all areas of the body that drain and filter infectious agents.
A cancer cell can travel through these systems. With time, the cancer cell can grow and form a new tumor that may not be near the primary site. When cancer spreads to another part of the body, it is called metastatic cancer.
Metastatic cancer is still named for the primary site. For example:
When cancer that started in the colon is found in the liver, it's called colon cancer that has metastasized to the liver. It is not called liver cancer.
Lung cancers are those that started from a lung cell. If the cancer spreads to the brain, it's called metastatic lung cancer, not brain cancer.
When cancer spreads to the nearby lymph nodes, those nodes are said to contain metastatic cancer. (Cancers that start in the lymph cells of a node are called lymphomas.)
If cancer cells are taken out from the metastatic tumor, they will look like the primary cancer cells. These metastatic tumors are also treated like the primary tumors. So colon cancer that has spread or metastasized to the liver is still treated like colon cancer is treated. The cells in the metastatic liver tumor look like the primary cancer cells in the colon. It's not liver cancer, it didn't start in liver cells. It's metastatic colon cancer.
Cancer can metastasize to any part of the body, but the most common sites are: